Modern aircraft typically receive instructions from air traffic control (ATC) or other control authorities during many phases of flight operations, including outbound taxi maneuvers, take-off, climb-out, cruise, descent, landing and inbound taxi maneuvers. The instructions typically include clearances (for example, clearances to land or ascend to a particular altitude) and/or other requests (for example, to tune the aircraft radio to a particular frequency). The instructions can be immediate or conditional. Immediate instructions are intended to be implemented and complied with immediately. Conditional instructions are not to be implemented until a particular condition is met. For example, some conditional instructions are not to be implemented until a specific time period has elapsed, or until the aircraft has reached a specified ground point or altitude.
Conditional instructions have the advantage of providing the aircraft crew with advance notice of a requested change for the path of the aircraft. However, conditional clearances may also pose problems. For example, the crew may not realize that the clearance is conditional and may accordingly implement the instruction prematurely. In other cases, the crew may lose track of when or where the instruction is to be implemented and may accordingly implement the instruction either prematurely or too late. Still further, some instructions include multiple conditional clearances (e.g., clearances that are to be implemented only after multiple conditions are met, or a series of clearances that are to be implemented sequentially as certain conditions are met). Such instructions can be ambiguous and therefore difficult for the crew to understand. These instructions can also be difficult for the crew to track and implement at the correct time and/or location. Some existing aircraft systems provide a warning to the crew if a particular clearance condition is violated. However, such systems may not address the foregoing problems in the most efficient and effective manner.